The invention is in the field of photoflash lamps of the type having a pair of lead-in wires carrying a filament or other electrical ignition means inside a bulb containing combustible material such as shredded metal foil and a combustion-supporting gas such as oxygen.
Flash lamps are conventionally constructed with a pair of lead-in wires extending through a seal region at the bottom of the bulb. Cameras and other apparatus for using flash lamps are arranged to normally position the lamps basedown when they are flashed, so that the hot burning metal particles will fall down onto the relatively thick seal region of the bulb and will be less likely to cause cracking of the bulb and explosion of the lamp than if they were to fall against the relatively thin sides or top of the bulb. Occasionally incompletely burned particles of metal (from either or both of the shredded foil and the lead-in wires) fall to the bottom of the bulb and cause shorts, or partial shorts, across the lead-in wires. This is undesirable, especially if more than one lamp is connected in parallel across the source of energy, because the shorted or partially shorted lamp draws and wastes electrical energy from the battery or other source of firing voltage. Also, certain electrical circuits which sequentially flash the lamps of a flash array, such as the FlashBar array now commercially available, require that the lamps have an open circuit, or at least a relatively high impedance compared to that of an unflashed lamp, internally across their lead-in wires after being flashed in order that the circuit can function to flash the next lamp in the array. Also, the flashability of certain types of lamps, such as those designed to be flashed by a high voltage, low current power source, may be impaired or prevented if the lead-in wires are shorted together by one or more strands of the combustible metal foil.
U.S. Pat. No. 3,816,054 to Baldrige and Sobieski discloses a flash lamp construction having a glass sleeve around one of the lead-in wires for reducing the likelihood of after-flash shorting. Another known technique is to enclose the lead-in wires with glass extending from the bottom of the bulb. U.S. Pat. No. 3,501,254 is an example of such a technique.